This invention relates to membrane switch circuit layouts and the method of manufacturing such. Specifically, it relates to membrane switch circuit layouts having multiple insulated membrane layers each having electrical circuit paths printed thereon. The invention provides electrical connection is desired at discrete positions between the circuit paths on different membrane layers.
The manufacture of membrane switch circuit layouts having thru-holes is presently performed with a screen-printing process. One method of screen-printing generally provides printing a membrane with an electrical circuit path on both its top and bottom surface. Thru-holes are selectively cut through the membrane where electrical connection is desired between the circuit path printed on the top surface and the circuit path printed on the bottom surface. Normally, the membrane is then placed with its bottom surface on a blotting paper or support paper. Conductive ink is pressed through the thru-holes from the top surface to the bottom surface. If a blotting or support paper is not used, excess ink must be wiped from the surface beneath the membrane. If a blotting or support paper is used, the paper must be replaced before the next step. The membrane is then turned over and placed with its top surface on a blotting or support paper. Conductive ink is pressed through the thru-holes from the bottom surface to the top surface to ensure complete filling of the holes. If a blotting or support paper is not used, excess ink must again be wiped from the surface beneath the membrane. This process is labor intensive and costly.
An additional concern with the current printing process is that the excess ink may spread out along the bottom or top surface of the membrane once the ink is pressed through the thru-hole. This causes a sloppy connection and may spread over multiple paths of the circuit trace on the bottom surface. In such a case, the entire membrane switch circuit must be reproduced.
Another problem with this process method for membrane switch circuit layouts is that the electrical connection between the circuits paths on the various surfaces may deteriorate. Deterioration results because there is no support of the conductive ink filled hole. Bending, pressure, or normal wear may result in the ink flaking from the hole and deteriorating the electrical connection.
Alternately, membrane switch circuit layouts may be printed using a dielectric or insulating layer between layers of circuit traces printed on one side of a membrane. That method involves printing a first conductive trace on a surface of the membrane; selectively printing a dielectric or insulating layer, a non-conductive ink, over the first conductive trace, leaving open areas where electrical conduction is desired between conductive traces; and printing a second conductive trace over the dielectric or insulating layer. Printing of the dielectric or insulating layer requires two printing passes to ensure no xe2x80x9cpinholexe2x80x9d gaps. Any pinhole in the dielectric or insulating layer may result in a shorted circuit. Further, under high humidity conditions, the dielectric or insulating layer absorbs moisture that provides an undesirable path for silver migration through the dielectric or insulating layer. Silver migration results in a high resistance short in the circuit.
Due, at least, to the two printing passes, this method is labor and cost intensive. The resulting membrane switch circuit similarly suffers from possible degradation of the traces. In particular, there is concern that the dielectric or insulating layer will deteriorate through bending, pressure, or normal wear. This would cause open communication between the two traces. It is similarly possible that the second conductive trace would deteriorate and cause a lack of electrical conductivity along the path desired.
To address the difficulties noted above, it is an object of this invention to provide a membrane switch circuit layout without the shortcomings of those in the current art and a more cost-effective method for manufacturing membrane switch circuit layouts. While reference is made explicitly to two membrane circuit layers, it should be apparent to those skilled in the art that the circuit may be manufactured of any number of membrane layers. The membrane switch circuit layout and method for producing such of the current invention eliminates the shortcomings of the prior art and provides a cost-effect method for producing the membrane switch circuit.
In a particularly preferred embodiment of the present invention, the membrane switch circuit comprises two membrane layers, a first membrane layer and second membrane layer. Each membrane layer is printed with a circuit trace. The second membrane layer has thru-holes selectively cut there through to provide electrical connection between the circuit trace on the first membrane layer and the circuit trace on the second membrane layer at discrete points. The second membrane layer is positioned over the first membrane layer and conductive ink is pressed through the thru-holes. Pads may be printed on the first membrane layer for receiving the conductive ink. Gravity forces the ink through the hole to contact the bottom layer. The spread of the ink is constrained to the size of the hole and there is little risk of the ink spreading along the bottom layer as it is blocked by the contact of the two membrane layers with one another. A viscous ink is preferred to enable the ink to completely fill the hole.
The first membrane layer, positioned beneath the second membrane layer, may provide support for the conductive ink. This prevents the need for use of blotting or support paper during the printing process. It decreases the printing process from two steps to one step as the hole is completely filled in one step rather than requiring flipping of the membrane layer and filling from the opposite surface. Additionally, the support provided by the first membrane layer demonstrably decreases the deterioration of the electrical connection between circuit paths provided by the conductive ink.
The second membrane layer additionally provides insulation between the conductive traces at all locations other than those where thru-holes have been selectively cut. This eliminates the necessity of an additional insulating layer for use when intersecting traces (e.g., crossovers) are not intended to be inter-conductive.
The invention may be further understood from the following more detailed description taken with the accompanying drawings.